1. Field of the Invention
The present invention relates to a method for polymerizing alpha-olefins, a catalyst for such a polymerization method and a method for producing such a catalyst. A particular aspect of the present invention relates to a method for preparing a catalyst which produces linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) having a relatively narrow molecular weight distribution, as evidenced by relatively low values of melt flow ratios (MFR), suitable for film and injection molding applications.
2. Description of the Prior Art
Linear low density polyethylene polymers possess properties which distinguish them from other polyethylene polymers, such as homopolymers of polyethylene. Certain of these properties are described in Anderson et al, U.S. Pat. No. 4,076,698.
Karol et al, U.S. Pat. No. 4,302,566, describe a process for producing certain linear low density polyethylene polymers in a gas phase, fluid bed reactor.
Graff, U.S. Pat. No. 4,173,547, Stevens et al, U.S. Pat. No. 3,787,384, Strobel et al, U.S. Pat. No. 4,148,754, and Ziegler, deceased, et al, U.S. Pat. No. 4,063,009, each describe various polymerization processes suitable for producing forms of polyethylene other than linear low density polyethylene, per se.
Graff, U.S. Pat. No. 4,173,547, describes a supported catalyst obtained by treating a support with both an organoaluminum compound and an organomagnesium compound followed by contacting this treated support with a tetravalent titanium compound.
Stevens et al, U.S. Pat. No. 3,787,384, and Strobel et al, U.S. Pat. No. 4,148,754, describe a catalyst prepared by first reacting a support (e.g., silica containing reactive hydroxyl groups) with an organomagnesium compound (e.g., a Grignard reagent) and then combining this reacted support with a tetravalent titanium compound. According to the teachings of both of these patents, no unreacted organomagnesium compound is present when the reacted support is contacted with the tetravalent titanium compound.
Ziegler, deceased, et al, U.S. Pat. No. 4,063,009, describe a catalyst which is the reaction product of an organomagnesium compound (e.g., an alkylmagnesium halide) with a tetravalent titanium compound. The reaction of the organomagnesium compound with the tetravalent titanium compound takes place in the absence of a support material.
A vanadium-containing catalyst, used in conjunction with triisobutylaluminum as a co-catalyst, is disclosed by W. L. Carrick et al in Journal of American Chemical Society, Volume 82, page 1502 (1960) and Volume 83, page 2654 (1961).
Nowlin et al, U.S. Pat. No. 4,481,301, the entire contents of which are incorporated herein by reference, disclose a supported alpha-olefin polymerization catalyst composition prepared by reacting a support containing OH groups with a stoichiometric excess of an organomagnesium composition, with respect to the OH groups content, and then reacting the product with a tetravalent titanium compound.
When the LLDPE or HDPE resins are fabricated into injection-molded products, it is imperative to assure that such products are not susceptible to warping or shrinking. As is known to those skilled in the art, the degree of warping or shrinking can be predicted from the molecular weight distribution of the resins. Resins having relatively narrow molecular weight distribution produce injection-molded products exhibiting a minimum amount of warping or shrinkage. Conversely, resins having relatively broad molecular weight distribution produce injection-molded products more likely to undergo warping or shrinkage. One of the measures of the molecular weight distribution of the resin is melt flow ratio (MFR), which is the ratio of high melt flow index (HLMI or I.sub.21) to melt index (I.sub.2) for a given resin. Resins having relatively low MFR values, e.g., of about 20 to about 50, have relatively narrow molecular weight distribution. Additionally, LLDPE resins having such relatively low MFR values produce films of better strength properties than resins with high MFR values.
Another important property of the resins used in injection molding applications is melt index (I.sub.2). As is also known to those skilled in the art, resins having relatively high I.sub.2 values, of e.g., 5-200, when melted in the extruder, have a relatively low viscosity. Low viscosity is important in such applications because it enables the molten resin to completely occupy the volume of the mold to consistently produce high quality injection molded products and minimize the number of faulty products.
It is possible, as is known to those skilled in the art, to increase the melt index of a resin by increasing the amount of hydrogen (H.sub.2) introduced into the polymerization reactor. However, the use of excessive amounts of hydrogen decreases the reactor volume available for the alpha-olefin reactants, especially ethylene, thereby reducing production rates.
Accordingly, it is important to provide a catalyst composition capable of producing alpha-olefin polymers having both, relatively narrow molecular weight distribution (low MFR values) and relatively high melt index (I.sub.2) without the use of large amounts of hydrogen.
It is a primary object of the present invention to prepare a high activity catalyst for the polymerization of alpha-olefins which yields products of a relatively narrow molecular weight distribution and relatively high melt index suitable for films and injection molding applications.
It is an additional object of the present invention to provide a catalytic process for polymerizing alpha-olefins which yields linear low density polyethylene of a relatively narrow molecular weight distribution.